The present invention relates to an electrostatic latent image forming apparatus of the uniplane control type which is capable of forming, by applying voltage pulses of opposite polarities to styluses and control electrodes which are arranged closely to one another in one plane, an electrostatic latent image on an electrostatic latent image recording medium formed by a dielectric layer or by a combination of a dielectric layer and at least one conductive layer, which medium is in contact with the styluses and the control electrodes.
FIG. 6 is a schematic sectional view of a conventional example of such uniplane control type electrostatic latent image forming apparatus, and FIG. 7 is an electric equivalent circuit diagram thereof.
In FIG. 6, reference numeral 11 denotes an electrostatic recording paper as an electrostatic latent image recording medium, which is formed by four layers including a surface electrostatic layer 1, an intermediate conductive layer 2, a paper base 3 and a back resistance layer (conductive layer) 4. Reference numeral 8 denotes a stylus, and numerals 7a and 7b denote control electrodes. Reference numeral 9 denotes a stylus drive circuit for applying to the stylus 8 a recording voltage pulse having a negative polarity, and numeral 10 denotes a control electrode drive circuit for applying to the control electrodes 7a and 7b a recording voltage pulse having a positive polarity. Reference numeral 6 denotes a pad roller used for pressing the electrostatic recording paper 11 against the stylus 8 and the control electrodes 7a and 7b, which pad roller is made of an insulating material (such as polyurethane foam).
The operation, function of each constituent, and characteristics of the conventional electrostatic latent image forming apparatus having the above construction will be described below with reference to FIG. 7.
In this electrostatic latent image forming apparatus, on the occasion of forming (recording) an electrostatic latent image, the stylus drive circuit 9 acts to apply a recording voltage pulse of a negative polarity to the stylus 8 and, at the same time, the control electrode drive circuit 10 acts to apply a recording voltage pulse of a positive polarity to the control the electrodes 7a and 7b.
Application of the positive polarity voltage to the control electrodes 7a and 7b raises electric potentials of the intermediate conductive layer 2, the paper base 3 and the back resistance layer 4 at positions thereof located directly below the control electrodes 7a and 7b due to capacitive coupling dependent on an electrostatic capacitance Cc possessed by the dielectric layer 1. The potentials of the intermediate conductive layer 2, the paper base 3 and the back resistance layer 4 are transferred to positions of the respective layers directly below the stylus 8 due to the existence of their respective resistivities Rc, Rp and Rb, and are then made to appear on the surface of the dielectric layer 1 owing to an electrostatic capacitance Cp possessed by the dielectric layer 1 at a position thereof directly below the stylus 8. In consequence, the difference in potential between the stylus 8 and the dielectric layer 1 at the position directly below the stylus 8 exceeds a predetermined value, so that discharge is caused to take place between the stylus 8 and the dielectric layer 1. Then, negative charge is transferred from the stylus 8 to the dielectric layer 1 and stored in the latter, thereby forming an electrostatic latent image on the electrostatic recording paper 11.
Each of the layers of the electrostatic recording paper 11 has the following function or service.
First, the surface dielectric layer 1 is made of a mixture of insulating thermoplastic resin and insulating pigment so that it serves as a charge carrier for holding charged particles for a long time and it serves to form a discharge gap of about 10 .mu.m between the stylus 8 and the control electrodes 7a and 7b with the aid of an insulating pigment having a diameter of about 3 to 6 .mu.m.
The function of the intermediate conductive layer 2 is to effectively concentrate the positive voltage applied to the control electrodes 7a and 7b in the dielectric layer 1 at a position thereof located directly below the stylus 8. This layer also serves as a liquid barrier so that, in coating the dielectric layer 1 on the paper base 3, the resin and the pigment are prevented from immersing into cavities in the paper base 3 to assure the coating with a uniform thickness.
The paper base 3 serves as a part for constituting a base on which the intermediate layer 2 and the dielectric layer 1 are formed and, at the same time, it usually contains water existing in equilibrium with the ambient humidity so as to perform the same function as the intermediate conductive layer 2.
Finally, the back resistance layer 4 does not directly take part in the formation of the electrostatic latent image like the intermediate layer 2, except that it serves to raise the potential at a position directly below the stylus 8. However, it contributes to prevention of any fog caused by the bias due to the rise in potentials of the intermediate layer 2 and the paper base 3 in forming the electrostatic latent image.
However, the above construction has suffered a problem that the formation of an electrostatic latent image cannot be performed stably at high or low humidities.
This is because the resistance values of the intermediate conductive layer, the paper base and the back resistance layer of the electrostatic recording paper vary in accordance with the humidity.
In other words, at high humidities, the resistance values of these layers are lowered to increase leakage occurring through the back resistance layer, so that the potential of the intermediate conductive layer at a position thereof directly below the stylus cannot increase readily, with the result that no sufficient discharge takes place. Further, in case a matrix is constituted by a plurality of styluses and control electrodes, there is caused a ghost image.
To the contrary, at low humidities, an increase in the resistance value hinders the potential of the intermediate conductive layer from rising at a position thereof directly below the stylus, resulting in an sufficient discharge.
To cope with the above problems, it has even been considered to form the intermediate conductive layer and the back resistance layer of the electrostatic recording paper of an electronic conductive material so as to be hardly affected by the humidity (water content). However, this gives rise to another problem that the cost of the electrostatic recording paper is increased.